1. State of the Art
U.S. Pat. No. 4,548,366 discloses a winding arrangement in which a contact roller (in contact with the outer surface of a filament package) is driven to apply a controlled force to the package surface while the speed of rotation of the roller is regulated by regulating the speed of rotation of the package.
U.S. Pat. No. 4,765,552 (corresponding to European Publication 0 254 944) discloses limitation of the controlled force to a range given by a motor torque for the contact roller between 0 and 1.5 Newton-centimeter per filament package. This latter specification is unclear in its explanation of the quoted range but the justification appears to relate either to avoidance of "small slips" which cause yarn quality variations or to avoidance of speed differentials giving tube damage at first contact of the roller with a bare bobbin tube.
German Document 35 13 796 proposes a drive system in which the package is driven on its circumference by a friction drive roll while thread from a traverse motion is laid on the package by an additional contact roll. The contact roll is driven to give a slight excess speed of the contact roll relative to the package. This is designated to enable control of thread tension.
U.S. Pat. No. 4,986,483 describes in some detail the problems discussed below (in the section "Problem Addressed") and proposes a combination of a drive system of the type discussed above with a special traverse cam device. The drive system is intended to be operated in a manner such as to avoid the transmission of circumferential force between the contact roller and the package for minimizing the generation of slip between the contact roller and the package.
German Document 41 26 392 describes a system to apply feedback control to the generation of the motor torque for the contact roller. The generated motor torque is related directly to the circumferential force transferred between the roller and the package.
The stated purpose of the arrangement according to German Document 41 26 392 is the achievement of control over the force transmitted at the interface between the roller and the package. By this means, slip at the interface is to be avoided. According to the German specification, slip is especially likely to occur when contact pressure is low and the system is subject to variations which risk an approach to or exceeding of the slip limit. Another stated purpose is to avoid occurrence of inhomogeneities over the period of winding a package.
2. Problem Addressed
The present invention addresses the problem of building a cylindrical cross-wound package of filament under conditions such that the threadline tension immediately upstream from the winder is at a level which, if the same threadline tension persisted through to the package, would cause package build problems before the desired package dimensions are achieved. In order to explain this statement further, a brief explanation of package build problems as related to thread tension, will be given.
The basic problem involved in building a cross-wound package arises from the traverse motion needed to move the thread in the axial direction to generate the winding angle. It is an inevitable characteristic of this motion that the thread travels relatively slowly in the (end) reversal regions as compared with the central package region. Many improvements have been proposed in the mechanisms generating the traverse motion in order to mitigate this problem and they have had considerable success. Their effect is not, however, to eliminate the problem but only to delay its appearance. Thus, by means of improvements made in traverse mechanisms, we have been able to build steadily larger packages (i.e. of larger diameter) over the years.
It follows from the relatively slow axial motion of the thread in the reversal regions that more thread material is deposited in the end regions of the package than in the central region thereof. This has two effects; namely:
1) sooner or later the outer surface of the package is no longer cylindrical--it takes on a "saddle-like" appearance with raised "shoulders" at its edges (see FIGS. 8 and 12). PA1 2) the density (and therefore the hardness) of the package in the edge regions is higher than the density of the package in its central region.
The contact roller has long been used as a device for mitigating the first effect. By means of the contact pressure applied by this roller, it is possible to flatten the shoulders to some extent. The flattening effect is limited by outward bulging of package ends (i.e., the package side walls) due to the applied pressure (see FIG. 13). Therefore, as previously indicated, sooner or later (as package diameter increases) shoulders will appear and when they reach a certain size they lead to unstable thread layers within the package and hence to problems in subsequent unwinding for further processing.
The second effect works together with the threadline tension to exaggerate the first effect. Because package density is lower in the central region, the package is more easily compressible in its central region than at its ends. The tension of the thread as it is wound into the package exerts a compressing effect on the underlying thread layers (and on the tube which forms the core of the package). The greater the thread tension, therefore, the greater the compressing effect and the more the central package region is squeezed in relative to the end regions.
It is not necessary to provide any solution for this latter problem within the winder itself if threadline tension can be influenced upstream from the winder. Modern filament production processes are, however, tending in the direction of simplifying upstream processing, thereby gradually eliminating possibilities for determining thread tension as the thread enters the winder. Furthermore, modern filament processing techniques are tending to generate steadily higher threadline tensions. For economic reasons, there is a demand for steadily larger packages. The winder manufacturer is therefore faced nowadays with the problem of converting "given" threadline conditions at the winder inlet into conditions which enable satisfactory package build (as regards package form) up to diameters of at least 500 mm. However, the formation of a saddle shape and axial bulging, as described above, limits the size of packages which can be built under given winding conditions.
For reasons outlined above, in most cases the problems arise from high threadline tensions at the winder inlet working through to high tension at the point of laydown in the package. In a relatively small, but important, class of cases, however, the opposite problem arises. The technology of the process in those cases is such that the thread tends to relax as it is wound. In such cases, it is necessary to increase thread tension in order to ensure a desired package build.